The present invention relates to tires and more precisely to the beads of tires.
The role of the beads of tires is known. It is to assure the "clamping" of the tire on the rim on which it is mounted. For this purpose, the shape of the bead of the tire is adapted to the shape of the rim for which it is designed; in other words, the outer side face of the bead as well as its seat (that is to say, its radially inner face) have a shape which corresponds substantially to the shape of the corresponding rim, if one neglects the slight differences for the purpose of assuring good clamping of the bead on the rim.
As to the shape of the inner side face, it corresponds more or less to the shape of the outer side face until encountering the seat of the bead in a zone which in the present description will be referred to as the inner edge of the bead, where there is generally present a rather sharp edge known as the "bead toe". The rubber which appears there is known as the "bead point".
Tires for passenger cars are mounted, as a rule, on single-piece rims having flanges which are substantially perpendicular to the axis of the rim and having seats which receive the bead. The seats have a frustoconical surface slightly inclined with respect to the axis of the rim (by an angle of about 5.degree.) and are bordered by a protuberance known as a "hump" having a central groove which is radially lower than the seats. The bead toe cooperates with this protuberance which is arranged on the rim in order to impart to the tire the ability to remain mounted on the rim even when its inflation pressure is very insufficient.
The role of the inner edge of the bead is therefor important for the proper holding of the tire mounted on its rim.
When the tire is to be mounted on its rim or removed from it, tools are used which come into engagement with the rim substantially at the level of the lower edge of bead. These tools apply deformations very locally on the bead of the tire, which deformations may be very substantial to the extent of damaging the bead.
This leads to the need to reinforce the point of the bead of the tire in order that it can withstand without damage the stresses which the bead must undergo during its use. For this, textile strips are used, which increases the cost price of the tire.
It is also known that the mounting and removal of the tire cause a swinging of the bead by rotation around its center, that is to say, approximately around the bead wire that it contains. The stresses resulting therefrom are greater the greater the stiffness in rotation of the bead. The stiffness will be greater when using bead wires of a shape which is further from a circular cross section or when the anchoring of the carcass of the tire comprises numerous reinforcements intended to increase the strength thereof.
The present invention relates, in particular, to tires which include a bead on each side of the tire equational plane, each bead being defined by a substantially frustoconical seat which is inclined slightly with respect to the axis of the tire when the bead is in mounted position, an inner edge, an inner side face and an outer side face, the bead being intended to be applied radially from above on the seat of a rim, the inner side wall being directed towards the inner cavity of the tire, the inner edge forming a transition between said seat and the said inner side wall, and having a stiffness of the bead in rotation which is greater than 5 mN/radian. This covers the very great majority of ordinary tires, and it is intended to exclude tires which are especially developed so that the bead is easy to swing, such as, for instance, collapsible tires (see, for instance, U.S. Pat. No. 4,057,091) or tires designed to be mounted on special rims without central mounting grooves (see, for instance, U.S. Pat. No. 4,768,573).